many graphic cards connected to pc but I only want to use one of them.
many hdd attached to linux box but I only need to access some of them.

I hope to power off any hardware connected to my box which I don't need to use them. But I hope I don't need to disconnect them from my Linux box.
I hope to power on any hardware attached to my box when I need them.

So a linux command/script which can control power to any hardware interface is very useful.

No matter it is pci-e, sata-2, nic or wifi card, software solution to power on/off them will bring a lot of convenience.

Ubuntu can do anything why I cannot power off my hdd just by a command.

Open box and disconnect them is not a good solution.

I need linux command line solution. any idea?